In order to be fermented by yeast, the food reserve of barley, starch must be converted by enzymes into simple sugars. Two enzymes, a- and b- amylases, carry out the conversion. The latter is present in barley – but the former is made only during germination of the grain.
Malting begins by immersing barley harvested at less than 12 percent moisture in water at 120 to 150C (550 + 600) for 40 to 50 hours. During this steeping period, the barley may be drained and given air rests, or the steep may be forcibly aerated. As the grain imbibes water, its volume increases by about 25 percent, and its moisture content reaches about 45 percents. A white roots heath, called a chit, breaks through the husk, and the chitted barley is then removed from the steep for germination.
Activated by water and oxygen, the root embryo of the barley corn secretes a plant hormone called gibberellic acid, which initiates the synthesis of a- amylase. The a- and b- amylases then convert the starch molecules of the corn into sugars that the embryo can use as food. Other enzymes, such as the proteases and b- glucanases, attack the cell wall around the starch grains, converting insoluble proteins and complex sugars (called glucans) into soluble amino acids and glucose.
Green malt is dried to remove most of the moisture, leaving 5 percent in lager and 2 percent in traditional ale malts. This process arrests enzyme activity but leaves 40 to 60 percent in an active state. Curing at higher temperatures promotes a reaction between amino acids and sugars to form melanoidins, which give both colour and flavour to malt.
In the first stage of kilning, a high flow of dry air at 500C (120F) for lager malt and 650C (1500F) for ale malt is maintained through a bed of green malt. This lowers the moisture content from 45 to 25 percent. A second stage of drying removes more firmly bound water, the temperature rising to 700 – 750 (1600 – 1700F) and the moisture content falling to 12 percent. In the final curing stage the temperature is raised to 750 – 900C (1700 – 1950F) for lager and 900 to 1050C for ale. The finished malt is then cooled and screened to remove root lets.
For efficient extraction with water, malt must be milled. Early milling processes used stones driven manually or by water or animal power, but modern brewing uses mechanically driven roller mills. The design of the mill and the gap between the rolls are important in obtaining the correct reduction in size of the malt. The object is to retain the husk relatively intact while breaking up the brittle modified starch into particles.
Mixing the Mash
The milled malt, called grist, is mixed with water, providing conditions in which starch, other molecules, and enzymes are dissolved and rapid enzyme action takes place. The solute-rich liquid produced in mashing is called the work. Traditionally, mashing may be of one of two distinct types. The simplest process, in fusion mashing, uses a well-modified malt, two to three volumes of water per volume of grist, a single vessel (called mashtun), and a single temperature in the range of 620 to 670C (1450 to 1500F). With well-modified malt, breakdown of proteins and glucans has already occurred at the malting stage, and at 650C the starch readily gelatinizes and the amylases become very active. Less well-modified malt, however, benefits from a period of mashing at lower temperatures to permit the breakdown of proteins and glucans. This requires some form of temperature programming, which is achieved by decoction mashing. After grist is mashed in at 350 to 400C (950 to 1050F), a proportion is removed, boiled and added back. Mashing with two or three of these decoctions raises the temperature in stages to 650C. The decoction process, traditional in lager brewing, uses four to six volumes of water per volume of grist and requires a second vessel called the mash cooker.
Other sources of starch that gelatinize at 550 to 650C can be mashed along with malt. Wheat flour and corn (maize) flakes may be added directly to the mash, whereas corn grits and rice grits must first be boiled in order to gelatinize. Their use requires a third vessel, the cereal cooker.
Modern mashing system use mixed grists and mash mixers, which are efficiently stirred and temperature-programmed mashing vessels. Enzymes of bacterial and fungal origin may be added as aids. Ale and lagers are mashed in the same equipment, but they require different temperature programs and grist composition. Modern breweries often practice high-gravity brewing, in which highly concentrated worts are made, fermented, and then diluted, allowing more beer to be brewed on the same equipment.
Separating the Wort
The mashtun used in infusion mashing is fitted with a false base containing precisely machined slots through which the husk, preserved during milling, cannot pass the trapped husk thus forms a filterbed that removes solids from the wort as it is drained, leaving a residue of spent grains. Wort separation takes four to sixteen hours. For through extraction, the solids are sprayed, or sparged, with water at 700C.
The decoction brewer transfers the mash into separation vessel called the lautertun, where a shallow fitter bed is formed, allowing a more rapid runoff time of about 2 ½ hours. Large modern breweries use either lautertuns or special mash filters to speed up the runoff and conduct 10 or 12 mashes a day. As much as 97percent of the soluble material is obtained, and 75 percent of this is fermentable. Wort is approximately 10 percent sugar, and it contains amino acids, salts, vitamins, carbohydrate and small amounts of protein.
Flavouring Value of Hops
Several varieties of the hop (Humulus lupulus) are selected and bred for the bitter and aromatic qualities that they lend to brewing. The female flowers, or cones, produce tiny glands that contain the chemicals of value in brewing. Humulones are the chemical constituents extracted during wort boiling. One fraction of these, the a-acids, is isomerized by heat to form the related iso-aacids, which are responsible for the characteristic bitter flavour of beer.
Traditionally the dried hop cones are added whole to the boiling wort, but powedered compressed hops are often used because they are more efficiently extracted. In addition, the hop components may be extracted by solvents such as liquid carbondioxide and added in this form to the wort or after isomerization, to the finished beer.
Heating and Cooling
The kettle boil lasts 60 to 90 minutes, sterilizing the wort, evaporating, undesirable aromas, and precipitating insoluble proteins. Trub and spent hops are then removed in a separator where the hop cones form the filter bed. In modern practice a more rapid whirl pool separator is also used. This devise is a cylindrical vessel into which wort is pumped at a tangent, the circulating whirlpool movement causing solids to form a cone at the bottom. Clarified wort is cooled, formerly in shallow troughs or by trickling down an inclined. Cooled plate but now in a plate heat exchanger. This last is an enclosed, hygienic vessel in which hot wort runs along plates while cold water passes along the other side in the opposite direction. Oxygen is added at this stage, and the cooled wort passes to fermentation vessels.
The simple sugars in wort are converted to alcohol and carbondioxide. Fermentation is carried out by yeast, which is added, or pitched, to the wort at three kilograms per hectolitre, yielding, 10,000,000 cells per millilitre of wort.
Brewing is unique among the fermentation industries in that yeast from one fermentation is used to pitch the next. This means that hygienic conditions and rigorous quality control are necessary. A high proportion of live cells and freedom from bacterial and other yeasts are important quality considerations.
Traditional open-topped earthenware fermentation vessels gave way to round, wooden and later square, copper-lined fermentors and brewery fermentation systems evolved around the mechanism used to separate yeast from freshly fermented, or green, beer. Top fermentations, in which yeast rises to the surface, require the most elaborate systems, but most brewing operations now use more hygienically operated closed vessels and bottom fermentation. These vessels, erected outside the brewery, are several thousand hectolitres in capacity (one hectolitre – 26 gallons) and are made of stainless steel. Temperature control is achieved automatically by circulating cold liquid in jackets fitted to the wall of the vessel.
The temperature of the wort at pitching is 150C to 180C for ale and 70 to 120C for lager. As fermentation proceeds, the specific gravity falls as the sugars are converted by the yeast. The extent of fermentation is governed by the wort composition and by the amount of fermentable sugar to remain in maturing beer. During fermentation, yeast multiples five to eight fold and generates heat. The temperature is allowed to rise, until it reaches 200 to 230C for ale and 120 to 170C for lager. At that point the fermentation is cooled to 150C for ale and 40C for lager, considerably slowing yeast action. Yeast is then removed and the green beer, still containing about 500,000 yeast cells per millilitre, is transferred to a conditioning or maturation vessels, where a secondary fermentation may take place. In traditional brewing, the primary stage of fermentation took seven days for ale and three weeks or more for lagers, these times haves been shortened to two to four days and seven to ten days by modern practices using more efficient fermentation vessels.
MATURATION AND PACKAGING
Priming and Krausening
A slow secondary fermentation of residual or added sugar (called primings), or in lager brewing, the addition of actively fermenting wort (called Krausen) generates carbondioxide, which is rented and purges the green beer of undesirable volatile compounds. Continued yeast activity also removes strong flavouring compounds such as diacetyl. Allowing pressure to build up in the sealed vessel then increases the level of carbonation, giving the beer its “condition”. In traditional brewing, large volumes of ale were conditioned. In tanks for seven days at 150C, whereas lagers were matured at 00C (320F) for up to three months. These long maturation periods were caused by the precipitation of protein tannin complexes, which at low temperatures form “chill hazes” that are slow in setting out. Modern practice speeds up this process by adding excess tannin, clarifying with protein or tanning adsorbent, or using enzymes to degrade the proteins.
Traditional, or “real”, ales are packaged into casks. Sugar primings, clarifying agents such as isinglass finings and whole hops are added and the beer is transferred to the point of sale, where it is carefully rented to the proper level of conditioning before being sold.
Beer produced on a large scale in modern breweries is kept free of oxygen, filtered through cellulose or diatomaceous earth to remove all yeast, and packaged at 00C under pressure of carbondioxide. Beer produced by high-gravity brewing is diluted to the desired alcohol concentration, immediately prior to packaging, with oxygen-free carbonated water. Most beers packaged in bottles or metal cans are pasteurized in pack by heating to 600C for five to 20 minutes. Beer is also packaged into metal cans are pasteurized in pack by heating into metal kegs of 50 litre capacity after pasteurization at 700C for five to 20 seconds. Modern packaging machinery is designed to operate hygienically, exclude air and run at rates of 2,000 cans or bottles per minute.